Graphviz: A Plugin for Pelican

Graphviz is a Pelican plugin that allows the inclusion of Graphviz images using the Markdown markup format. The code for the Graphviz figure is included as a block in the article’s source. In the output HTML file, the Graphviz figure can appear as either a <svg> or embedded into a <img> element using the Base64 format.

Installation

This plugin can be installed via:

python -m pip install pelican-graphviz

As long as you have not explicitly added a PLUGINS setting to your Pelican settings file, then the newly-installed plugin should be automatically detected and enabled. Otherwise, you must add graphviz to your existing PLUGINS list. For more information, please see the How to Use Plugins documentation.

Graphviz must be installed on the system, otherwise this plugin will be deactivated. Graphviz can be installed on Debian-based systems via:

sudo aptitude install graphviz

For macOS, Graphviz can be installed via Homebrew:

brew install graphviz

Usage

In the Markdown source, the Graphviz code must be inserted as an individual block (i.e., separated from the rest of the material by blank lines), like the following:

..graphviz dot
digraph G {
  graph [rankdir = LR];
  Hello -> World
}

This will insert an image in your article like this:

The block must start with ..graphviz (this is configurable — see below). The word dot in the first line indicates the program that will be run to produce the image. The available programs are: dot, neato, twopi, circo, fdp, sfdp, and patchwork (see the Graphviz documentation for details). The Graphviz code must start in the second line of the block. Notice that newlines are not allowed inside the Graphviz block.

Styling with CSS

The image is generated in HTML with an <img> element inside an HTML element (by default a <div>, but this is configurable — see below). The latter has class graphviz (this is also configurable — see below). A possible CSS styling would be:

div.graphviz  {
    margin: 10px;
}
div.graphviz img {
    display: block;
    padding: 5px;
    margin-left: auto;
    margin-right: auto;
    text-align: center;
    border-style: solid;
    border-width: 1px;
    border-color: rgb(192, 177, 177);
    -webkit-border-radius: 5px;
}

Configuration

The following variables can be set in the Pelican settings file:

• GRAPHVIZ_HTML_ELEMENT: The HTML inside which the generated Graphviz image is inserted (defaults to 'div'; another sensible possibility would be 'span').

• GRAPHVIZ_IMAGE_CLASS: Class of the <div> element including the generated Graphviz image (defaults to 'graphviz').

• GRAPHVIZ_BLOCK_START: Starting tag for the Graphviz block in Markdown (defaults to '..graphviz').

• GRAPHVIZ_COMPRESS: Compress the resulting SVG XML to an image (defaults to True). Without compression, more SVG features are available, for instance including clickable URLs inside the Graphviz diagram.

• GRAPHVIZ_ALT_TEXT: String that will be used as the default value for the alt property of the generated <img> HTML element (defaults to "[GRAPH]"). It is only meaningful when the reuslting SVG output is compressed.

The values above can be overridden for each individual block using the syntax below:

..graphviz [key1=val1, key2="val2"...] dot

The allowed keys are html-element, image-class, block-start, alt-text, and compress. For the latter, the value can be either yes or no.

If the value needs to include a comma (,) or an equal sign (=), then use the key2="val2" form.

Output Image Format

The format of the embedded image is SVG, and there is currently no way to change it. This format was chosen over others (like PNG) for two reasons. First, the generated SRC string in Base64 seem to be shorter for SVG than for PNG. Second, the image will be available in the browser in a high-quality vectorized format. As a caveat, notice that this choice may prevent display in browsers lacking proper SVG support.

Text alternative for the image

When generating compressed SVG images (the default), an <img> element will appear in the HTML output. According to the Web Content Accessibility Guidelines, non-text content, like <img> elements, should have an text alternative. The graphviz plugin complies with this recommendation and always generate a alt property for the generated <img> element. The value of the alt property will be, in order of priority:

1. The value of the Graphviz block option alt-text,
2. the ID of the Graphviz element ("G" in the example above), or
3. the value of the global configuration variable GRAPHVIZ_ALT_TEXT.

Alternatives

An alternative to this plugin is the Graphviz tag provided by the Liquid Tags plugin, which differs from this plugin in several respects. First, the Liquid Tags version uses the syntax {% graphviz { <program> […] } %}, while this Graphviz plugin uses the Markdown extension syntax ..graphviz <program> […]. Regarding the rendered output, the differences are:

• Both plugins output an element with class="graphviz". However, only the Graphviz plugin allows you to change the class name via a configuration variable (GRAPHVIZ_IMAGE_CLASS).
• Liquid Tags encodes the image as src="data:image/png;base64,[…]", while the Graphviz plugin encodes it as src="data:image/svg+xml;base64,[…]". This has two impacts. First, the size of the HTML code produced by this Graphviz plugin is much smaller. For example, the Base64 string for the Graphviz code digraph graphname {a -> b -> c; b -> d;} is four times smaller. Second, the Liquid Tags version generates a raster image file, whereas this Graphviz plugin produces a vector image that can be zoomed without image quality loss.
• Liquid Tags outputs the Graphviz image inside a <span>, whereas this Graphviz plugin offers a choice for the container element.

To-Do

Contributions that make this plugin work with reStructuredText content are welcome.

Contributing

Contributions are welcome and much appreciated. Every little bit helps. You can contribute by improving the documentation, adding missing features, and fixing bugs. You can also help out by reviewing and commenting on existing issues.

To start contributing to this plugin, review the Contributing to Pelican documentation, beginning with the Contributing Code section.

Acknowledgments

Thanks to Justin Mayer, for helping with migration of this plugin under the Pelican Plugins organization, and to Maxim Kochurov, for introducing the GRAPHVIZ_COMPRESS configuration variable.

Thanks to weeheavy for suggesting the addition of the alt property to the HTML <img> generated element (issue #30).

Author

Copyright © 2015, 2021, 2023–2025 Rafael Laboissière rafael@laboissiere.net

License

This project is licensed under the terms of the AGPL license, version 3.0 or later.